658 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



It was long ago noticed that if the skin be covered with a thick 

 varnish death will be rapidly produced. The most probable explanation 

 is that in addition to the rapid cooling of the body by the dilatation of 

 the cutaneous capillaries the suppression of perspiration causes the 

 retention in the economy of some poisonous principle, for the general 

 symptoms closely resemble those of poisoning — the respiration becomes 

 slow and disturbed, the pulsation of the heart reduced in frequency, 

 and convulsions frequently accompany the final stages. 



5. The Lachrymal Secretion. — The lachrymal glands belong to 

 the group .of compound racemose glands, and secrete a fluid which is 

 tolerably rich in albuminous constituents. The tears are a colorless 

 liquid of salty taste and alkaline reaction. They contain about 1 per 

 cent, of solids, which consist of a small amount of mucus and albumen, 

 coagulable by heat, and traces of fat and mineral salts. Of the latter 

 sodium^ chloride is the principal representative, with a small quantity of 

 alkaline and earthy salts. 



The following table represents their analyses : — 



"Water, .... 982.00 



Albumen and traces of mucus, 5.00 



Sodium chloride, . . 13.00 



Other inorganic salts, ... 0.2 



The lachrymal secretion is continuous and is produced by the direct 

 protoplasmic activity of the secretory cells of this gland : the blood 

 pressure is of special influence, and it is probable that the lachrymation 

 which accompanies laughing, coughing, vomiting, etc., is produced by 

 local increase of pressure through the arrest of the venous circulation. 



The lachrymal secretion, like that formed bj' the other glands, is 

 under the control of the nervous system. Normally the lachrymal secre- 

 tion is of reflex origin, the afferent impulse being conducted either from 

 the conjunctiva or nasal fossae, over the first and second branches of the 

 trigeminal nerve, from a retinal stimulation, as by intense light, or through 

 some mental impression. As a rule, such stimuli lead to an increased 

 secretion of the glands on both sides, with the exception of stimuli 

 originating in the nasal fossae or conjunctiva, in which case the secretion 

 is unilateral. The secretorjr nerve is the lachrymal nerve. Its stimula- 

 tion is followed by an abundant secretion of tears, while its section is 

 followed after a time by a continuous secretion corresponding to the 

 paralytic secretion which follows section of the chorda tympani. 



Normally the tears, after passing over the anterior surface of the 

 eyeball, partially evaporate and partially are conducted through the 

 lachrymal passages to the nose, only when in excess overflowing the 

 lower eyelid and running over the cheeks. The function of the tears is 

 to protect the eye by keeping its surface moist and to wash away foreign 

 bodies. 



